The present invention relates to apparatus and methods for determining the density of materials by radiation backscatter, and in particular relates to an improved apparatus and method by which an accurate determination of density can be obtained even though the test material may have a rough or irregular surface.
Nuclear radiation gauges have been widely used for measuring the density of soil and asphaltic materials. Such gauges typically include a source of gamma radiation which is located adjacent the surface of the test material and which directs gamma radiation into the test material, and a radiation detector also located adjacent the surface of the test material for detecting radiation which is reflected or scattered back to the surface. From this detector reading a determination of the density of the material can be made. Such gauges are commonly referred to as "backscatter" gauges.
The gauges commonly in use for measuring the density of soil, asphalt and other materials are most effective at measuring densities of materials over depths of approximately four to six inches. However, with the increasing cost of paving materials, the practice in maintaining and resurfacing paved roadbeds has become one of applying relatively thin layers or overlays one to two inches thick. With layers of such a thickness range, prior density gauges are ineffective for measuring the density of the overlay applied since the density reading which is obtained from such gauges reflects not only the density of the thin layer but also the density of the underlying base material.
Accordingly, efforts have been made to devise a method and device which is capable of accurately determining the density of relatively thin layers of material applied to an underlying base material.
One method which has been developed for measuring the density of thin layers involves the use of a nomograph. By this technique the density of the composite, the density of the underlying base and the thickness of the overlay are separately measured, and the density of the thin overlay is determined by reference to the nomograph. A similar approach, but employing an equation rather than a nomograph, is described in U.S. Pat. No. 4,389,126. These techniques have several shortcomings, most significantly including the necessity of obtaining a density measurement of the base material before application of the thin overlay.
In commonly-owned U.S. patent application Ser. No. 477,820 filed Mar. 22, 1983 entitled "Radiation Scatter Apparatus and Method," U.S. Pat. No. 4,525,854 and in commonly-owned U.S. patent application Ser. No. 681,302, U.S. Pat. No. 4,641,030 filed concurrently herewith and entitled "Apparatus and Method for Directly Measuring the Density of a Thin Layer," there are disclosed apparatus and methods which are capable of directly measuring the density of a thin layer of material without the necessity of making multiple separate density measurements. The apparatus and methods described in the aforementioned commonly-owned copending applications rely upon the use of multiple detector systems for collecting independent sets of data from which the density of the thin top layer can be directly determined.
It has been previously recognized that when making density measurements by the radiation backscatter method, the accuracy is greatest when the surface of the test material is relatively smooth. Rough surfaces tend to produce a low density reading, due to the presence of surface voids or irregularities. This problem becomes particularly troublesome with density measurements of thin layers. Since a density determination of a thin top layer represents a relatively small sample of the material close to the surface, it follows that surface voids and irregularities have a greater influence on the density reading. Consequently, the need exists for a reliable technique to eliminate or correct for inaccuracy due to the presence of surface voids and irregularities when making density measurements of materials which have an irregular or rough surface, especially density measurements of thin layers.
In a study conducted by the California Department of Transportation (CALTRANS) reported in Alexander, M. L. et al "California Study of Asphalt Concrete Density Measurement-Nuclear Versus Core Density," Placement and Compaction of Asphalt Mixtures, ASTM STP 829, F. T. Wagner, Ed., American Society for Testing and Materials, 1984, pp. 80-92; the problem of accurately measuring the density of thin surface layers was addressed, and several possible measurement techniques were considered.
One specific approach which was described in this publication was to insert a rubber pad of known density and thickness between a conventional nuclear density gauge and the pavement. The density of the top portion of the pavement could then be determined by measuring the composite density of the pavement and pad and then adjusting for the influence of the pad. The study concluded, however, that the use of a pad of known density and thickness as an interlayer between the gauge and the surface provided no apparent advantage when determining the density of thin layers, and this technique was, therefore, not recommended.